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                        <h1 id="how-do-i-implement-an-existing-com-interface"><a class="header" href="#how-do-i-implement-an-existing-com-interface">How do I implement an existing COM interface?</a></h1>
<p>In some cases, you may need to implement an existing COM interface rather than simply calling an existing implementation provided by the operating system. This is where the <code>implement</code> feature and macro come in handy. The <a href="https://crates.io/crates/windows">windows</a> crate provides optional implementation support hidden behind the <code>implement</code> feature. Once enabled, the <a href="https://docs.rs/windows-implement/latest/windows_implement/attr.implement.html">implement</a> macro may be used to implement any number of COM interfaces. The macro takes care of implementing <code>IUnknown</code> itself.</p>
<p>Let's implement a simple interface defined by Windows to illustrate. The <code>IPersist</code> interface is defined in the <code>Win32::System::Com</code> module, so we'll start by adding a dependency on the <code>windows</code> crate and include the <code>Win32_System_Com</code> feature:</p>
<pre><code class="language-toml">[dependencies.windows]
version = "0.52"
features = [
    "implement",
    "Win32_System_Com",
]
</code></pre>
<p>The <code>implement</code> feature unlocks the implementation support.</p>
<p>The <code>implement</code> macro is included by the <code>windows::core</code> module so we'll keep things simple by including it all as follows:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use windows::{core::*, Win32::System::Com::*};
<span class="boring">}</span></code></pre></pre>
<p>Now its time for the implementation:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[implement(IPersist)]
struct Persist(GUID);
<span class="boring">}</span></code></pre></pre>
<p>The <code>implement</code> macro will provide the necessary implementation for the <code>IUnknown</code> interface's lifetime management and interface discovery for whatever interfaces are included in the attribute. In this case, only <code>IPersist</code> is to be implemented.</p>
<p>The implementation itself is defined by a trait that follows the <code>&lt;interface name&gt;_Impl</code> pattern and its up to us to implement it for our implementation as follows:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl IPersist_Impl for Persist_Impl {
    fn GetClassID(&amp;self) -&gt; Result&lt;GUID&gt; {
        Ok(self.0)
    }
}
<span class="boring">}</span></code></pre></pre>
<p>The IPersist interface, originally <a href="https://learn.microsoft.com/en-us/windows/win32/api/objidl/nn-objidl-ipersist">documented here</a>, has a single method that returns a <code>GUID</code>, so we'll just implement it by returning the value contained within our implementation. The <code>window</code> crate and <code>implement</code> macro will take care of the rest by providing the actual COM virtual function call and virtual function table layout needed to turn this into a heap-allocated and reference-counted COM object.</p>
<p>All that remains is to move, or box, the implementation into the COM implementation provided by the <code>implement</code> macro through the <code>Into</code> trait:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let guid = GUID::new()?;
let persist: IPersist = Persist(guid).into();
<span class="boring">}</span></code></pre></pre>
<p>At this point, we can simply treat <code>persist</code> as the COM object that it is:</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let guid2 = unsafe { persist.GetClassID()? };
assert_eq!(guid, guid2);
println!("{:?}", guid);
<span class="boring">}</span></code></pre></pre>
<p>Here's a <a href="https://github.com/microsoft/windows-rs/tree/master/crates/samples/windows/bits">complete example</a>.</p>

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